Particulate magnetic recording media and method of manufacture thereof

ABSTRACT

A magnetic recording medium is fabricated by the electrodeposition of acicular magnetic particles onto an electrically conductive substrate from a mixture that is continuously dispersed and circulated through the electrodeposition vessel. Electrodeposition is effected at a critical electric field strength so that the particles are aligned in solution, and deposited perpendicular to the substrate surface such that the medium is highly densified. By use of anisotropic particles the resultant medium is adaptable for perpendicular recording, and by use of isotropic particles the resultant medium will support both longitudinal and perpendicular recording.

This is a division of application Ser. No. 900,210, filed Aug. 25, 1986,now U.S. Pat. No. 4,778,719.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to magnetic recording media utilizing particulatemagnetic material, and in particular to media having both increasedconcentrations of magnetic particles and a preferred particleorientation.

The invention, as well as the prior art, will be described withreference to the figures of which:

FIGS. 1, 2, 3, 4 are illustrations of geometrical concepts useful inunderstanding both the prior art and the invention.

FIG. 5 is a schematic representation of longitudinal magnetic recordingknown in the prior art,

FIGS. 6, 7 are schematic representations of longitudinal magnetic diskrecording known in the prior art,

FIG. 8 is an illustration of perpendicular magnetic recording known inthe prior art.

FIGS. 9a, 9b, 10a, 10b are illustrations useful in understanding onepurpose of the invention,

FIGS. 11, 12 are illustrations of the electrodeposition of magneticcoating according to the prior art,

FIG. 13 is a graph useful in understanding the principle of theinvention,

FIGS. 14a, 14b are illustrations of particle alignment useful inunderstanding the invention, and

FIG. 15 is a block diagram of electrodeposition apparatus according tothe invention.

2. DESCRIPTION RELATIVE TO THE PRIOR ART

Attention is initially directed to U.S. Pat. No. 4,578,280 issued Mar.25, 1986 to Greiner et al, which discloses state of the art magneticmedia useful for perpendicular magnetic recording, and to U.S. Pat. No.4,585,535 issued Apr. 29, 1986 to Sher et al which discloses anelectrophoretic method of producing magnetic recording media.

The continuing trend in digital magnetic recording is towards increaseddata density recorded on the magnetic medium. This requirementtranslates into both the recording of more tracks per inch and therecording of shorter signal wavelengths, resulting in higher areal datapacking densities. Additionally, in modern digital recording, it isnecessary that the media accommodate the capability to overwritepreviously recorded data; that is, new data recorded over old data musteffectively erase the previously recorded data. Overwrite is a complexphenomenon, but it is known in the art that the thinner the magneticmedium the more effective the overwriting of previously recorded longwavelengths by newly recorded short wavelengths. The data recordingsignals, based on the particular digital encoding method employed,generally have frequency components extending over several octaves. Theoverwrite (i.e. erase) capability must extend, therefore, over theentire frequency spectrum of t he recorded signal. Inadequate erasure ofthe long wavelength signal results in a remanent long wavelengthmagnetization of the medium which interacts with the newly recordedsignals causing unwanted pulse crowding and peak shift. A solution tothis problem has been the reduction of the physical thickness of therecording medium with the goal of restricting the medium thickness todepths that can be adequately overwritten by a short wavelength signal.A price is paid, however, for this solution to the problem. The signalamplitude read from the medium during playback is proportional to themagnetization of the medium, which in turn is proportional to the totalvolume of magnetized material. By restricting the thickness of themagnetic coating the volume of magnetizable material is reduced, and thesignal amplitude is degraded.

A variety of techniques have been employed in the prior art to addressthe volume of magnetizable material problem. In the production of media,the most common fabrication technique involves the coating ofparticulate magnetic material in suspension with a binder and solventonto a substrate material. The solvent is driven off during a dryingprocess and the medium is then compacted or "calendered" to increase thedensity of the magnetic layer. The final density attained is, to a largedegree, dependent upon two factors: First, because the coating processinvolves the flow of the particle-binder-solvent mixture onto thesubstrate, rheological parameters controlling the dynamics of this flowestablish the minimal amount of solvent needed for stable flow. When thesolvent is driven off during drying, voids remain; the more solventutilized, the greater the proportion of voids. While calendering reducesthe effects of the voids, the calendering does not compact the materialso as to completely eliminate the effects of the voids. Second, thegeometric form of the particle itself sets a limit on the density whichmay be attained even if the compacting were 100 percent efficient inremoving the voids. For example, considering a dispersion of identicalspherical particles, the maximum packing fraction [the total volume ofthe spheres divided by the total volume of a cube enclosing them,expressed either as a decimal or percentage]has a theoretical maximum of0.72. No amount of calendering can compact a medium consisting ofidentical spherical particles to a greater density.

FIG. 1 shows the hexagonal close-packed structure of sphericalparticles, and the inevitable typical interstices 10, 12, 14 which giverise to the above mentioned packing fraction limitation of 0.72.Generally, magnetic recording particles are not spherical but areneedle-like, or acicular, in shape. Such particles typically have aspectratios, i.e. the ratio of the length of the major axis to that of aminor axis of the particle, ranging from two to seven. For simplicity,it is advantageous to consider acicular particles of ideal crosssection, i.e. uniform cross sections which are either rectangles orcircles for the entire length of the particle. Considering the idealcase of acicular particles having a rectangular cross section andaligned as shown in FIG. 2, the resultant structure has a packingfraction of 1.0. If particles of cylindrical cross section are randomlyarranged in a coating, larger interstices 13, 15 [FIG. 3]will inevitablyoccur with attendantly reduced packing fraction. On the other hand, ifsuch particles were perfectly aligned as shown in FIG. 4, they wouldhave a packing fraction of 0.91 due to the typical interstices 16, 18,20 arising from the circular cross sections. While actual particlesapproximate the aforementioned shapes, in reality, magnetic particlestend to be cigar-shaped and of irregular cross section. In practice,considerably lower packing fractions, of the order of 0.4, are realizedin prior art fabrication of coated magnetic tapes and disks; the reasonsbeing the presence of voids, the fact the particles are actuallycigar-shaped, and the problem of not being able to completely align theparticles.

It is desirable, at this point, to describe briefly the technique knownin the art for measuring the packing fraction of particulate magneticmedia. The saturation magnetization of a known volume of the magneticmedium is measured by means of a vibrating sample magnetometer. Thesaturation magnetization is directly proportional to the volume ofmagnetizable material in the sample; the greater the volume ofmagnetizable particles relative to the volume of binder or voids (suchas those due to particle misalignment), the higher the measuredsaturation magnetization. Data is also available in the art for the"intrinsic" saturation magnetization values for the magnetic materialunder consideration; that is, the saturation magnetization of thematerial, not in particle form, but as a continuous atomic crystallinestructure. This may be visualized as a homogeneous bar of the basicmagnetic material; it is the most dense configuration in which the givenmagnetic material could exist. The ratio of the saturation magnetizationper unit volume of the sample to the intrinsic saturation magnetizationof the material is defined as the packig fraction of the particulatemedium, and it will be appreciated that the packing fraction is adefinitive measure of the density of recordable material present in themedium.

The desirability of alignment of acicular particles in the mediumrelates both to the resultant increased density of particles, and alsoto the parameters of the recording process itself. Magnetic particlesexhibit "shape anisotropy," that is, the preference for particlemagnetization to occur along a particular geometric dimension of theparticle. An acicular particle generally sustains magnetization alongthe particle's long or major axis, and therefore the particle'sorientation directly impacts the effectiveness of the recording process.The relation between particle orientation and the recording process maybe appreciated by consideration of the most common technique ofrecording...longitudinal recording. In FIG. 5, longitudinal recording isperformed by a recording head 22 which is in contact with a movingmagnetic medium 24. A magnetic field 26, generated at the gap 28 of thehead 22, is applied longitudinally with respect to the medium 24. Thefield 26 magnetizes the particles in the medium 24 in the direction ofthe field with a resultant remanent magnetization 30 in the medium. Whencurrent in the winding 32 of the head 22 reverses direction, the field26 reverses as does the direction of magnetization 34 in the medium. Inview of the previously described particle recording characteristic, thefabrication of magnetic media for use in longitudinal recording mandatesthat all the acicular particles lie essentially parallel to thedirection of the longitudinal field. It will be appreciated this alsoconforms to the geometric requirement for increasing the density ofmagnetic material in the coating by improving the packing fraction. Toaccomplish this during tape manufacturing a magnetic field is applied tothe coating to align the particles before the coating is dried.[Jorgensen, F., "The Complete Handbook of Magnetic Recording", BlueRidge Summit, Pa.: Tab Books, 1980, p. 38]. Under the action of thefield, the acicular particles rotate against the resisting couple due tothe viscosity of the binder-solvent mixture until their major axes arealigned with the direction of the applied field. Even with theapplication of magnetic fields, however, the packing fractions attainedare approximately only one half the theoretical values due to theappreciable viscosity of the coating mixtures, and the voids anddeviations from ideal particle shapes previously described.

In the manufacture of magnetic webs for the fabrication of magneticfloppy disks, the goal of increased packing density is further limitedby the shape anisotropy of the typical acicular particle. It will beappreciated that if the above described procedure was followed infabricating a web of magnetic material for disk application, theresultant disk would have all the particles aligned in one direction.Such a disk, 36, for use in longitudinal digital recording isillustrated in FIG. 6. The arrows 35 show the direction of alignment ofthe major axes of the particles, which, as previously stated, is alsothe preferred direction of magnetization due to shape anisotropy. Thelongitudinally oriented recording field 38 of a record/playbaack head 34operating on the disk 36, would sometimes be aligned with the particles'major axes, [FIG. 6], and one quarter of a disk rotational cycle later[FIG. 7]would be oriented perpendicular to the particles' major axes.The result is a continually varying orientation between the head fielddirection and the particle axes as the disk rotates. This produces avariation in magnetization, causing a "twice around" variation in theamplitude and phase of the recorded playback signal. To avoid theparticle alignment responsible for this unwanted signal modulation, theparticles are subjected to an intense a.c. magnetic field as they arecoated onto the web substrate. This field randomizes the directions ofthe axes of the particles and obviates the "twice around" problem. Thepenalty, however, is that the disk has a relatively low packing fractiondue to the resultant random orientation of the particles, andattendantly, the density and available signal are reduced. The relativemagnetic moment in the desired direction is also only about half that ofthe aligned particle value, further reducing the signal.

It is to be noted that the previous discussion of longitudinal recordingand its associated media specifies that the recording field is parallelto the plane of the medium and that the particles are aligned in theplane of the medium. In the magnetic recording art, an area of intensecurrent interest is that of "perpendicular" recording where, by way ofcontrast, the recording field is perpendicular to the plane of themedium. Perpendicular recording is illustrated in FIG. 8, where a singlepole head 48 is positioned above a medium 50 having the ability tosupport magnetization in a direction perpendicular to the plane of themedium 50. Current through the winding 53 of the head 48 generates amagnetic field 56 which penetrates the medium 50 perpendicular to itsplane and magnetizes the medium 50 in the field direction. The remanentmagnetization 52 is thus perpendicular to the plane of the medium. Inthe prior art, particulate perpendicular recording media has generallynot been available, and perpendicular recording has been restricted tosuch practices as those using cobalt-chromium alloys sputtered onto asubstrate. It is known that such alloys exhibit vertical anisotropy, andcurrent perpendicular recording has been virtually confined to the useof such alloys. The recognized problem of providing a "particulatemedium for perpendicular recording" is referred to by White, Robert M.(Editor), "Introduction to Magnetic Recording", New York, N.Y.; IEEEPress, 1985, p.69 wherein it is stated:

S. Iwasaki of Tohoku University has suggested that data might be storedperpendicular to the plane of the disk. Here the end of the core of ahead, and therefore a pole of a dipole magnetic field, confronts thedisk and throws field lines deep into the magnetic medium. As a resultthe data are stored, so to speak, on end. More than 100,000 magneticreversals per inch might be possible, but the implementation awaits thedevelopment of both the head and the medium for it. By the techniquedescribed above the magnetic dipoles of iron oxide can be oriented inthe plane of the disk. The problem is to find a way in which suchdipoles can be oriented perpendicular to the plane of the disk.

As will appear below, the present invention teaches how to so orient theparticles and, unlike the processes disclosed in U.S. Pat. No. 4,578,280and U.S. Pat. No. 4,585,535, does not require an auxiliary magneticfield to so align the particles perpendicular to the substrate.

The impetus to the application of perpendicular recording may beappreciated by reference to FIGS. 9a, 9b, 10a, 10b. FIG. 9a illustratesa longitudinally recorded transition 54. When a transition from onemagnetic orientation to another occurs, the longitudinally alignedrecorded cells interact in the manner of small bar magnets asillustrated in FIG. 9b. In this orientation, the bar magnets tend todemagnetize each other because each magnet's field bucks that of theother. The demagnetization effect is more pronounced for short magnets,or equivalently, in a recorded medium, for short recorded wavelengths[Jorgensen, supra p. 52]. On the other hand, in perpendicular recording,as illustrted in FIG. 10a, the fields of adjacent recorded transitions55 interact in the manner of bar magnets as shown in FIG. 10b. In thiscase the fields of the bar magnets aid rather than oppose each other.For this reason perpendicular recording is of great interest in shortwavelength recording technology where the demagnetizing effect presentin longitudinal recording is most manifest. Currently availablelongitudinal recording system attain a recording density ofapproximately 10,000 flux reversals per inch, while it is anticipated,as previously mentioned, that by use of perpendicular recording morethan 100,000 flux reversals per inch may be possible.

In addition to the previously described web coating process, it is alsoknown in the prior art to produce magnetic coatings byelectrodeposition. This technique is an application of the methodutilizing the phenomenon of "electrophoresis" which is exploited in thebroad field of electropainting. Electropainting has been applied to awide variety of painting requirements, from coating the insides of metalcans to painting automobile bodies. [Yeates, R. L. "Electropainting"Teddington, Great Britain: Robert Draper, Ltd., 1966]. In JapanesePatent Publication Number 25321/1977 entitled "Electrodeposition Paintfor Magnetic Recording", a mixture of magnetic particles in a watersoluble acyrilic polycarbonate resin emulsified uniformly in water isdescribed for use in the electrodeposition of a magnetic coating.

The Japanese technique is illustrated in FIG. 11. A vessel 42 contains amixture 43 of solvent, binder and magnetic particles. A cathode 44 andan anode 46 are immersed in the mixture 43, and a d.c. voltage source 48is connected between the cathode 44 and the anode 46. Each particletends to acquire an electric charge due to the interface between theparticle and the binder-solvent solution. A diffuse layer of charge thensurrounds the charged particle, so that the combination of chargedparticle and the diffuse layer is electrically neutral. The particlemoves under the electric force due to the field between the cathode andanode, and the diffuse layer tends to move with the particle to which itis attached but, at the same time, the diffuse layer is electricallyattracted to the other electrode. As the particle moves, new chargebuilds in front of the particle in the direction of motion, and thediffuse layer dissipates in the opposite direction. [Bockris, J. M., andReddy, A. K. N. "Modern Electrochemistry", New York: Plenum Press, 1974,Vol. 2, p. 833]. In the previously referenced Japanese PatentPublication, the electric field magnitude is indicated to beapproximately 17 volts/cm.: a field sufficient to move the magneticparticles to the electrode where they deposit as a magnetic coatig 41[FIG. 12].

SUMMARY OF THE INVENTION

In the electrodeposition of acicular particles to form a magneticcoating, rather than use of an electric field of just sufficientstrength to deposit the particles on the substrate, the inventionteaches the use of a field strength of sufficient magnitude to alsoalign the particles with the field. It has been discovered that inincreasing the field strength, a point is reached which is dramaticallycharacterized by a rapid and significant increase of density of theelectrodeposited medium. This field strength is defined as the "criticalfield strength", and the resultant medium, characteristic of depositionat this field strength, has a volumetric packing fraction greater than0.5, and typically one on the order of 0.9. Expanding on this point, itis hypothesized that at the critical field strength, the electric fieldnot only drives the particles to the electrode on which they deposit,but also rotates the particles so that their major axes are in thedirection of the electric field. This rotation occurs because the fieldexerts a torque on the charged particles proportional to the anglebetween the major axes and the field direction. The field, which is akinto that associated with a parallel plate capacitor, is essentiallyuniform between the electrodes. Because the directions of the electricfield lines are always perpendicular to the conductive electrodesurface, the particles are deposited on the electrode, which is also themedium substrate, with the particles' major axes perpendicular to theelectrode and aligned parallel to each other. The graph of FIG. 13illustrates the discovered dependence of packing fraction for acicularparticles as a function of electric field strength. It will beappreciated that there is a broad span of lower value field strengthswhere the field is of insufficient magnitude to completely align theparticles against the couple exerted by the viscose medium, and at theselower field strengths the particles arrive at the cathode with somewhatrandom orientations. As previously hypothesized, when the particles arecompletely aligned with the field the increase in coating densityoccurs; if the particles are only partially aligned they will stillrandomly pile up on the cathode [FIG. 14a]. In the presentimplementation of the invention, the graph of FIG. 13 indicates that itis not until the field reaches the critical strength of approximately325 volts/cm that complete particle alignment [FIG. 14b], with anattendant increased packing fraction, abruptly occurs.

The alignment of the major axes in the coating provides, as previouslyexplained, a highly densified magnetic layer. By control of thedeposition time a thin but dense magnetic layer with desirable overwritecharacteristics for digital recording is obtained. It will be seen that,unlike coatings earlier described where the major axes of the particleslie essentially parallel to the plane of the substrate, in theelectrodeposite coating obtained by following the teachings of theinvention, the major axes are aligned perpendicular to the substrate.Recalling that the direction of preferred magnetization for ananisotropic particle is along the particle major axis, it will beappreciated that such an electrodeposited layer using anisotropicparticles is advantageously suitable for perpendicular recording.

The teaching of the invention may also be applied to the fabrication ofa magnetic coating useful not only for perpendicular recording but alsosuitable for longitudinal recording as well, the resultant longitudinalrecording occurring even though the major axes of the acicular particlesare perpendicular to the direction of the longitudinal recording fieldlines. To implement such a coating, cobalt doped acicular particles, ofthe type described in French Patent No. 2,129,841 and French Patent No.2,199,155, are deposited on a substrate in accordance with the teachingsof the present invention. Such doped acicular particles have theproperty of supporting magnetization not only along their major axes butalong their minor axes as well. This embodiment of the invention maytherefore be applied to provide an improved solution to the "twicearound" problem of longitudinal disk recording previously described. Amagnetic disk fabricated from such isotropic particles in accordancewith the teachings of the invention will comprise a highly denserecording surface of parallel acicular particles oriented perpendicularto the disk substrate. The thickness of the magnetic coating may beconcurrently controlled to provide optimum overwrite characteristics.Because these particles are isotropic, the medium will supportlongitudinal recording as well as perpendicular recording, and since theparticles are magnetizable in the directions of the minor axes for allrotational positions of the disk, the "twice around" effect iseliminated.

DESCRIPTION OF THE INVENTION

The practice of the invention may be further understood by reference toFIG. 15. A motor 50 drives a "sand mill" 52 of a type well known in theart, i.e. the "Mini Motor Mill" manufactured by Eiger Machinery Corp.,Chicago, Ill. Small glass beads are employed in the continuouslyoperating sand mill 52 to finely disperse a mixture 53 of solvent,binder, magnetic particles and additive which, for example, are employedin the following percentages by weight:

    ______________________________________                                        Cyclohexanone as solvent                                                                          80.5%                                                     Magnetic particles  16.1                                                      Polyurethane as binder                                                                            2.3                                                       Surfactant          1.1                                                       ______________________________________                                    

The surfactant is a complex phosphate ester free acid of the typemanufactured by GAF Corp., Wayne, N.J., under the trade name "Gafac R.E.610". This surfactant is an anionic surfactant which not only enhancesthe amount of charge present on the magnetic particle, but also controlsthe sign of the charge. In the present embodiment the particles arecharged positively. A cationic surfactant would change the sign of thecharge, with the result of depositing the magnetic coating not on thecathode but on the anode. The polyurethane binder is available under thetrade name "Nippolan" from Nippon Urethane Co. Ltd., Japan.

The mixture is pumped from the sand mill 52 by means of a pump 54 into avessel 56 in which the electrodeposition takes place. Within the vessel56, a disk anode 58 and an equi-area disk cathode 60 (the latter servingas the substrate for the magnetic coating), are mounted on anonconductive standoff 62 with a nonconductive spacer 64 separating theanode 58 and the cathode 60. The spacer 64 provides a separation of 1.33cm and is located at the central area of the cathode 60; an area whichrequires no magnetic coating because it is not addressed duringrecording. The mixture flow is continuous, and the mixture 53 returns tothe sand mill 52 by means of a return line 66. A d.c. voltage source 68of approximately 432 volts is connected between the anode 58 and cathode60 to effect the electrodeposition at a field intensity of approximately325 volts/cm. The current flow in the mixture is derived primarily fromthe ionic double layer previously described, and the mixture has a highspecific resistance in the range of from 10⁶ ≠10⁹ ohm-cm. As a result,the current is limited by the resistance of the mixture; the mixturebeing substantially higher resistance than the coating being deposited,and deposition continues at constant current even as the coating buildsup on the cathode. The deposition time is approximately 5 secs.,providing a coating of approximately 10 microns thick. The coatedcathode 60 is then rinsed, oven dried to drive off any adherent solvent,and the resultant surface is the magnetic medium configured as a disk.

UNLIKE THE TEACHINGS OF U.S. PAT. NOS. 4,578,280 AND 4,585,535, WHEREINTHE PARTICLE ALIGNMENT IS ACCOMPLISHED BY EXTERNAL MAGNETIC FIELDSAPPLIED WHEN THE PARTICLES ARE ALREADY CAPTIVE ON THE SUBSTRATE, ANDWHEN THEY ARE ESSENTIALLY ENCAPSULATED IN A SEMI-SOLIDIFIED BINDER, THEPRESENT INVENTION TEACHES ALIGNING THE PARTICLES WHILE THEY ARE STILLMOBILE IN SOLUTION AND DEPOSITING THEM ON THE SUBSTRATE IN AN ALIGNEDCONDITION BY MEANS OF THE SAME ELECTRIC FIELD THAT PERFORMS THEALIGNMENT. AS A RESULT OF THE "IN-FLUID" ALIGNMENT OF PARTICLES, MEDIAMADE PURSUANT TO THE INVENTION IS CHARACTERIZED--AND MAY BE SOIDENTIFIED--BY:

(1) PACKING FRACTIONS AS HIGH AS 0.9 ARE ATTAINED BY ALIGNMENT INSOLUTION, AND

(2) THE DEPOSITED MEDIUM IS OF UNIFORM PACKING DENSITY THROUGHOUT ITSENTIRE VOLUME, HAVING BEEN DEPOSITED FROM A UNIFORM HOMOGENEOUS SOLUTIONOF ALIGNED PARTICLES.

ADDITIONAL EXAMPLES FOR THE PRACTICE OF THE INVENTION

1. Solvent: tetrahydrofuran, (THF), a readily available organic solvent.

Binder: DeSolite 2764-70-130 acrylate, available from De Soto Corp., DesPlaines, Ill.

Surfactant: Emcol CC55, quarternary ammonium acetate, available fromWitco Chemical Co., New York, N.Y.

2. Solvent: Acetone, a readily available organic solvent.

Binder: Nippolan, polyurethane, available from Nippon Urethane IndustryCo., Ltd., Japan.

Surfactant: Emcol CC55, quarternary ammonium acetate, available fromWitco Chemical Co., New York, N.Y.

3. Solvent: Methylisobutylketone (MIBK), a readily available organicsolvent.

Binder: DeSolite 2764-70-130 acrylate, available from De Soto Corp., DesPlaines, Ill.

Surfactant: Gafac R.E. 610, phosphate ester free acid, available fromthe GAF Corp., Wayne, N.J.

4. Solvent: Methylethylketone (MEK), a readily available organicsolvent.

Binder: Nippolan, polyurethane, available from Nippon Urethane IndustryCo., Ltd., Japan.

Surfactant: Gafac R.E. 610, phosphate ester free acid, available fromGAF Corp., Wayne, N.J.

Ratios for the respective above indicated mixtures may be selectedwithin a wide variety of individual combinations, each of which isassociated with a respective critical voltage.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention. For example, the cathode disk upon which the medium iselectrodeposited may be replaced with a continuously moving flexible webhaving a conductive surface partially immersed in the mixture, and theanode disk may be replaced by a surface opposed to the web so that acontrolled electric field exists between the web and the anode. Acoating may be continually electrodeposited, followed by appropriaterinsing and drying procedures. In this embodmient of the invention,magnetic media in the form of magnetic tape is fabricated by theelectrodeposition technique of the invention.

What is claimed is:
 1. A method to provide a magnetic medium by theelectrodeposition of acicular magnetic particles onto a substrate havinga conductive surface, said method comprising:a. dispersing saidparticles in a liquid to provide a dispersion wherein said particlesacquire charge at the interface between said liquid and said particles.b. immersing said substrate and an electrode in said dispersion, and c.generating a d.c. electric field at said surface of said substrate byapplying a d.c. voltage between said electrode and said surface, saidfield having an intensity substantially equal to the critical fieldintensity for aligning said particles in the direction of said field. 2.In a system for the electrodeposition of magnetic particles on asubstrate to form a magnetic recording medium, said system being of thetype wherein a d.c. electric field is established between electrodesimmersed in a liquid containing said magnetic particles, whereby saidparticles are electrodeposited on one of said electrodes to form saidmagnetic recording medium, the improvement wherein said liquid andmagnetic particles constitute a dispersion of acicular magneticparticles in a mixture of solvent, binder and surfactant having aspecific resistance substantially in the range of 10⁶ to 10⁹ ohm-cm,with the electrical resistance of said dispersion being substantiallygreater than the electrical resistance of said deposited medium duringsaid electrodeposition, whereby said electrodeposition occurs atessentially constant current.